The present invention relates generally to MR imaging and, more particularly, to a method and apparatus of reducing image intensity variation during MR acquisition of volumetric data during passage of an intravascular contrast agent. More specifically, the present invention relates to a method and apparatus of reducing intensity variations in a reconstructed image that result from relatively large gradients being applied across an imaging volume.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Time Resolved Imaging of Contrast Kinetics using Elliptical Centric View Ordering (ECTRICKS) is an imaging technique for MR angiography that combines variable rate k-space sampling and view sharing to acquire 3D volumetric data rapidly during the passage of a contrast agent through the vasculature of a patient. One advantage of ECTRICKS imaging is that arterial-venous separation as well as artifact suppression is improved in time resolved MR angiography and, in particular, angiographic images in carotid arteries and peripheral vessels. ECTRICKS imaging has recently been applied to monitor contrast uptake in cancer tissues, for example, in the breast. In this regard, it has been shown that analysis of the contrast uptake using compartment-based pharmacokinetic models may differentiate between cancerous tissues and healthy tissues. Further, the contrast uptake may also be analyzed to ascertain and differentiate between different types of cancers. While ECTRICKS provides improved temporal resolution when compared to multi-phase 3D fast gradient echo acquisition, eddy current and gradient driver hysteresis associated with large amplitude gradients used to acquire data for peripheral regions of k-space causes intensity variations.
Referring to FIG. 4, an ECTRICKS acquisition for a number of imaging frames 77 is shown wherein the MR signal for the center of k-space is sampled more frequently than for outer or peripheral regions of k-space. For example, k-space data 76 may be partitioned into four, but equivalently sized regions A-D. The regions are divided by elliptical contour lines that represent the distance to the center of k-space. Since most of the signal comes from the center of k-space, i.e. region-A, this region is sampled more frequently than peripheral regions B, C, and D. During image reconstruction, linear interpolation is typically implemented to synthesize missing regions at any given point in time. For example, to reconstruct volumetric images of time frame 13, the A-region at frame 13 is used, the linear interpolation (B′) of B-regions at frames 12 and 18 is used for the missing B-region data, the linear interpolation (C′) of C-regions at frames 8 and 14 is used for the missing C-region data, and the linear interpolation (D′) of D-regions at frames 10 and 16 is used for the missing D-region data. It should be noted that it is customary for contrast enhanced MR angiography to acquire a non-contrast mask volume (frames 1-4) to enable complex subtraction during reconstruction thereby allowing for background subtracted vessel-only images 78 to be generated.
To acquire encoded data for the outer or peripheral regions of k-space (regions B-D in FIG. 4), phase/slice encoding gradient pulses of larger amplitudes are used relative to that used to acquire data for the center of k-space (region A). To generate the larger amplitude gradients, a larger amount of current is used during the acquisition of the outer regions compared to the current used for the inner regions. For example, a much larger current is used during acquisition of the D-region compared to that used for acquisition of the A-region. These relatively large amounts of current induce eddy currents that cause distortion when the MR signal is sampled for filling the center of k-space. As a result, complex effects appear on the image that include ghosting and image intensity changes.
As noted above, these image intensity variations may be caused by eddy current and gradient driver hysteresis associated with gradient pulses with large amplitudes used to acquire data for peripheral regions of k-space. With ECTRICKS acquisition the center of k-space is collected or sampled immediately after each sampling of a non-center of k-space region, as illustrated in FIG. 4. The intensity variations in the center of k-space can be upwards of approximately 15 percent which is detrimental to analysis based on temporal intensity changes such as contrast uptake curve analysis. That is, the variation, which is a systematic variation, appears as part of the overall noise and thus introduces error in any index parameter that a model based approach would try to extract. Additionally, because of the complexity of the variation, it is difficult to alleviate this error using only post-processing techniques.
It would therefore be desirable to have a system and method capable of reducing image intensity variations during imaging acquisitions that employ large encoding gradients immediately prior to sampling of a center of k-space.